The present invention relates to electromagnetic radiation sources, and more particularly to a method and apparatus for producing high intensity electromagnetic radiation.
Arc lamps are well known and have many industrial and scientific applications, ranging from simulation of sunlight to rapid thermal processing in the manufacture of semiconductor computer chips, for example. An arc lamp typically includes a cylindrical quartz tube filled with an inert gas such as xenon or argon, for example. A plasma arc is generated and sustained between a pair of electrodes spaced axially apart within the tube. Conventional arc lamps of this nature are widely available with power outputs of up to 10 to 20 kilowatts, for example, roughly two orders of magnitude more powerful than a conventional filament lamp.
More recently, improving upon such arc lamps, a new generation of high intensity arc lamps has been produced, such as the water wall arc lamps manufactured by Vortek Industries Ltd. of Vancouver, Canada. In such a water wall arc lamp, a high pressure water flow, typically seven atmospheres for example, is circulated in a vortex motion along an inner surface of the cylindrical arc tube, to cool the tube. The vortex motion of the water results in more efficient cooling of the tube and also minimizes or eliminates splashing which might otherwise interfere with the arc. It has been found that water wall arc lamps of this nature are capable of achieving much higher power outputs than arc lamps which lack such an outer wall. For example, typical water wall arc lamps manufactured by Vortek Industries Ltd. range from 200 kilowatt to 500 kilowatt output and custom arc lamps having power outputs of 1.5 megawatts or greater are also available.
However, high intensity arc lamps of this nature pose special problems that do not significantly affect other, less powerful arc lamps. Many applications require electromagnetic radiation with a specific spectral distribution that differs from the emission spectrum of an arc lamp. For example, designing and testing of solar energy cells require simulated sunlight, which typically requires a general reshaping of the arc lamp""s emission spectrum as well as a reduction in the relative intensity of strong lines in the arc lamp spectrum. Typical low power arc lamps provide optical filters to filter undesirable wavelengths from the arc radiation spectrum to obtain a filtered spectrum which more closely approximates sunlight. One sunlight simulation system supplied by Atlas Electric Devices Company of Chicago, U.S.A., uses xenon as an inert gas because this gas generates an arc that is similar to sunlight. Undesirable wavelengths are filtered from the arc spectrum by use of transparent selective absorption materials such as glass, quartz or borosilicate glass, for example. However, absorption of undesirable wavelengths also heats the material.
For relatively low power applications involving 10 to 20 kilowatt arc lamps, for example, use of absorption filters is appropriate as the relatively small amount of heat generated by this absorption can be removed at a reasonable cost. However, absorption filtering is not practical for existing water wall arc lamps or other high intensity arc lamps, as the absorbing materials become significantly overheated. A removal of such additional heat is difficult or impractical to achieve, and the useful lifetime of the absorption filtering materials is greatly reduced due to the increased thermal stress to which they are subjected. In addition, absorption filtering arc lamps require an initial xe2x80x9cagingxe2x80x9d process during which time the lamp cannot be used for accurate work because the radiation characteristics vary greatly. Although the radiation characteristics eventually stabilize to some extent, they nevertheless continue to vary and thus, even substituting xe2x80x9cpre-agedxe2x80x9d absorption filtering lamps detracts from usefulness of the lamp.
As an alternative to absorption filtering, reflective coatings have been applied to relatively low power arc lamps, to act as partial reflection optical filters. Such filters serve to pass desirable wavelengths through the filter, and reflect undesirable wavelengths back into the arc chamber, thus reducing heat build-up and waste of energy by absorption filtering. Partially reflecting filters of this nature may include xe2x80x9csemi-silveredxe2x80x9d, vapor-deposited metallic film filters, or interference filters formed by special compounds deposited on the transparent material of the arc tube or on separate filter glasses. However, such reflective coatings are not suitable for existing water wall arc lamps or other high intensity arc lamps. Any type of reflection filtering is imperfect and some radiation is always absorbed by the filters, and also by the glass or quartz through which the radiation passes. For example, when a reflective coating is applied to an outer surface of an arc tube of an existing 500 kilowatt water wall arc lamp, the reflective coating would quickly begin to burn off due to the large amount of heat resulting from partial absorption of the arc lamp radiation. Such overheating and burning of the reflective coating would interfere with its filtering characteristics and would result in an extremely short useful lifetime. In addition, the increased heat would significantly increase the thermal stress on the arc tube itself, thereby significantly reducing the useful lifetime of the tube.
More generally, even in the absence of absorption or reflection filtering, arc tubes in high intensity arc lamps such as the aforementioned water wall arc lamps are generally subjected to much higher thermal and pressure stresses than corresponding tubes on relatively low power arc lamps.
Accordingly, there is a need for a way to obtain a desired high intensity output spectrum from a high intensity arc lamp.
The present invention addresses the above need by providing methods and apparatus for producing high intensity electromagnetic radiation. One such method involves generating a high power plasma arc between first and second electrodes of a high intensity arc lamp having an inner envelope and having a first flow of liquid along an inside surface of the inner envelope, the arc emitting the radiation. The method further involves producing a second flow of liquid in contact with an outside surface of the inner envelope. This preferably involves directing the second flow of liquid through a cooling chamber defined between the outside surface of the inner envelope and an inside surface of an outer envelope surrounding the inner envelope. Thus, both the inside and outside surfaces of the inner envelope are exposed to respective flows of liquid, thereby improving cooling of the inner envelope. This reduces the thermal stress on the inner envelope, thereby increasing its useful lifetime. In addition, this improved cooling allows for reflective coatings such as interference filters or other partially reflecting optical filters to be applied to the outside surface of the inner envelope without quickly burning off or otherwise deteriorating. Consequently, the invention allows reflective coatings to be used on high intensity arc lamps to enable a desired, high intensity spectrum to be produced.
The first flow of liquid on the inside surface of the inner envelope may be operated at a first pressure and the second flow of liquid on the outside surface of the inner envelope may be operated at a second pressure selected to achieve a desired balance between a first pressure gradient across the inner envelope and a second pressure gradient across the outer envelope. Because the arc chamber is typically pressurized at a relatively high pressure, such as seven atmospheres for example, this allows for the pressure load on the inner envelope to be significantly reduced.
According to another aspect of the invention, there is provided an apparatus for producing high intensity electromagnetic radiation. The apparatus includes a high intensity arc lamp having an inner envelope cooled by a first flow of liquid along an inside surface of the inner envelope and having first and second electrodes for generating a high power plasma arc within the inner envelope, the arc emitting the radiation. The apparatus further includes a cooling device for producing a second flow of liquid in contact with an outside surface of the inner envelope. The apparatus may further include an outer envelope surrounding the inner envelope to define a cooling chamber in a space between the outside surface of the inner envelope and an inside surface of the outer envelope.
Preferably, the apparatus further includes an energy redistributor for redistributing energy within a first radiation spectrum generated by the arc to produce a second radiation spectrum. The energy redistributor may include a partially reflecting optical filter for reflecting a first portion of energy at a first waveband centered about a strong line of the first radiation spectrum back into the arc, such that at least some of the first portion of energy is re-emitted at a second wavelength outside the first waveband. This effect, which was not previously feasible with high intensity arc lamps due to the overheating and burning of such filters, has been found to be particularly advantageous in such high power arc lamps. A significant amount of energy at undesirable wavelengths may thus be absorbed by the plasma arc and re-emitted in one of two ways. Some such absorbed energy is thermalized by the arc, or in other words, the temperature of the arc increases, thereby increasing the intensity across the entire spectrum of radiation emitted by the arc. Other portions of such absorbed energy are shifted from strong lines onto weak lines of the arc""s emission spectrum. Thus, energy at undesirable wavelengths may be effectively shifted to both desirable wavelengths and to somewhat less undesirable wavelengths.
In accordance with another aspect of the invention there is provided an envelope assembly for a high intensity radiation apparatus. The assembly includes an inner envelope having an inside surface defining in part an arc chamber, and an outer envelope enclosing the inner envelope, the inner and outer envelopes defining in part therebetween a cooling chamber. The assembly further includes inlet and outlet spacers cooperating with inlet and outlet end portions of the inner and outer envelopes respectively to provide the cooling chamber extending therebetween, the spacers having conduits to conduct cooling liquid relative to the cooling chamber.